Before I answer the question, I want to problematize the information you have been told about the Norse afterlife. The information that there were three separate realms of the dead is given by Snorri Sturluson, writing around 1220, more than 200 years after the conversion of Iceland to Christianity! He quotes the much older poem Grímnismál in saying that "Freyja chooses half of the slain, and Óðinn chooses the other." However, Hel doesn't seem to fit into this construction, and Snorri does a lot of interpretive work to attempt to make it fit. He points out Náströnd, the deepest part of Hel, where supposedly the wicked go and are constantly mauled by Níðhöggr. This may suggest that he has heard of multiple contradictory belief practices and is trying to reconcile them into a coherent system, or alternatively that he is trying to force pre-Christian oral traditions into a mold somewhat closer to Christianity. This is still a topic that is debated. u/steelcan909 has gone much more in depth on some of the reasons to distrust the Eddas here recently, as has u/Platypuskeeper here. It's a popular topic among the AskHistorians Norse scholars...

Even at the most generous interpretation, the version of beliefs outlined in the Eddas are derived mostly from an elite Norwegian practice. Pre-Christian Nordic religions are exceptionally heterogeneous, focused on local pockets of practice, and so it is very, very difficult to say anything about Norse peoples' writ large.

All that being said, child mortality. Those unreliable written sources do not mention what happens to the spirits of deceased children. So, we need to look at burial archaeology for evidence.

Most of the time, children were buried with things of value, such as a burial of a young Gotlandish girl who was buried with a horse and a dog (animal sacrifices in graves are usually reserved for extremely high-status graves). Generally, objects of cultural significance, such as rattles, may be included in infant graves, indicating a high degree of care and mourning, and potentially some kind of belief in an afterlife (Jóhanna Katrín Friðriksdóttir, Valkyrie: the Women of the Viking World, chapter 1.)

The most tantalizing bit of evidence comes from graves of young boys, who were buried with functioning weapons (see Lindqvist, M. 2004. Barn på vikingatiden. Gotländsk Arkiv 76: 74–77.) These mostly are found in ScotlandThis indicates a kind of symbolic warrior status, even though the weapons often measure 50% or more of their height. There aren't many graves like this, but there are enough to indicate that children could access the afterlife and status of a warrior, whether that's Valhalla or something else, despite not being able to fight themselves.